Generally, an obstacle detection device for a vehicle can be provided with an ultrasound oscillator. For example, as disclosed in JP-62-240890A, a vibration surface of the ultrasound oscillator fixedly contacts a predetermined portion of a back surface (vehicle inner side surface) of a vehicle bumper by bonding through an adhesive. Thus, the vibration surface of the ultrasound oscillator is integrated with the back surface of the bumper. Therefore, the bumper (including a contact surface thereof which contacts vibration surface of ultrasound oscillator) is partially used as a vibration member, so that ultrasound from the ultrasound oscillator is transferred to the exterior of the vehicle.
In this case, an opening end of a protection casing is fixed to the back surface of the bumper, in such a manner that the ultrasound oscillator which is fixed to the back surface of the bumper is covered by the protection casing. That is, the ultrasound oscillator is housed in the casing to be protected.
An ultrasound oscillator which is expandable contractible in a diametric direction thereof (i.e., direction which is parallel to back surface of bumper) can be used. In this case, when the ultrasound oscillator diametrical-direction expands/contracts, there will occur a distortion at the contact surface (which contacts ultrasound oscillator) of the back surface of the bumper because the surrounding of the ultrasound oscillator is fixed via the protection casing.
Thus, the vibration of the ultrasound oscillator is transferred to a broad range of the bumper from the contact surface of the bumper due to the distortion. Therefore, ultrasound is sent from the broad range of the bumper surface. Accordingly, the directivity of ultrasound which is sent is inhomogeneous and has peak and dip, so that an obstacle around the vehicle cannot be detected stably.
On the other hand, an ultrasound oscillator which is expandable/contractible in a thickness direction thereof (i.e., direction which is perpendicular to back surface of bumper) can be also used. In this case, it is capable to restrict the vibration of the ultrasound oscillator from being transferred to the broad range of the bumper and obtain a desirable directivity. The frequency of ultrasound from the ultrasound oscillator (thickness oscillator) is determined by the thickness thereof. That is, the ultrasound frequency will become low with an increase of the thickness of the ultrasound oscillator (i.e., become high with decrease of thickness).
When the thickness oscillator is used to detect the obstacle around the vehicle, it is desirable that the directivity of the thickness oscillator is wide. Thus, it is necessary to lower the ultrasound frequency (that is, increase thickness of thickness oscillator).
However, when the thickness of the thickness oscillator is increased, the weight of the thickness oscillator will increase. Therefore, it is difficult to make the thickness oscillator tightly contact the back surface of the bumper only by bonding through the adhesive. In this case, the vibration of the thickness oscillator is not substantially transferred to the back surface of the bumper.